What a retatrutide half-life of about 6 days means
Retatrutide (LY3437943) has a half-life of about 6 days, measured in an Eli Lilly phase 1b trial in people with type 2 diabetes. This page explains what that number means, why the published phase 1 and phase 2 trials used once-weekly dosing, which part of the molecule keeps it in circulation, and how it compares with semaglutide and tirzepatide. For the wider picture, see the <a href="/retatrutide">retatrutide research hub</a>.
| Compound | Half-life | Source | Dosing interval | Side chain and albumin binding | 4–5 half-lives (near steady state) |
|---|---|---|---|---|---|
| Retatrutide (investigational, not approved) | ≈6 days (≈5–7 days after single doses) | Phase 1b trial, 12 weeks, type 2 diabetes (Urva et al., Lancet 2022); single-dose study in healthy volunteers (Lilly protocol NCT04881760) | Once weekly in trials | 39 amino acids + C20 fatty diacid; albumin binding not reported | ≈24–30 days |
| Tirzepatide (Mounjaro) | ≈5 days | FDA label (Mounjaro) | Once weekly (label) | C20 fatty diacid; 99% albumin-bound | ≈20–25 days |
| Semaglutide (Ozempic, Wegovy) | ≈1 week | FDA labels (Ozempic, Wegovy) | Once weekly (injection labels) | C18 fatty di-acid; >99% albumin-bound | ≈4–5 weeks |
On this page
What is the half-life of retatrutide?
Retatrutide has a half-life of approximately 6 days, according to the phase 1b trial by Urva and colleagues in The Lancet (2022). Half-life is the time it takes for the amount of a substance in blood plasma to fall by half. In plain terms, about 6 days after a given point, half of the retatrutide present has been cleared.
In that trial, adults with type 2 diabetes received once-weekly retatrutide or placebo for 12 weeks. The investigators found that exposure rose in proportion to the amount administered. An earlier Lilly study gave single doses to healthy volunteers and measured a mean terminal half-life of about 5 to 7 days, as summarised in the phase 2 protocol.
Half-life describes clearance, not effect. It tells you how fast the molecule leaves the blood, not how long a biological response lasts in a given study.
Why did retatrutide trials use once-weekly dosing?
Retatrutide was given once a week in its trials because its half-life of about 6 days keeps it in circulation between administrations. Urva and colleagues concluded that its pharmacokinetics suggest suitability for once-weekly dosing. Coskun and colleagues reported in Cell Metabolism that the single-dose profile from the first human study supported the same interval.
The phase 2 obesity trial (Jastreboff et al., NEJM 2023) kept that interval: once-weekly dosing for 48 weeks, with gradual escalation every 4 weeks in some arms. Retatrutide remains investigational. It is not approved by the EMA, the FDA or any other regulator, and no approved dose exists anywhere.
What makes the retatrutide half-life so long?
Retatrutide stays in the blood for days because its 39-amino-acid peptide chain is attached to a C20 fatty diacid, a long fatty acid side chain. The phase 1b paper says this design gives an extended pharmacokinetic half-life while keeping the intended activity at the GLP-1, GIP and glucagon receptors.
The labels of related drugs explain how such side chains work. The FDA label for tirzepatide, another Lilly peptide, states that its C20 fatty diacid enables albumin binding and prolongs the half-life; tirzepatide is 99% bound to plasma albumin. The semaglutide labels name albumin binding through a C18 fatty di-acid as its main protraction mechanism, which lowers clearance by the kidneys and protects the peptide from breakdown.
The retatrutide papers we reviewed give no albumin-binding figure, so this mechanism is inferred from the shared design, not measured.
When does retatrutide reach steady state?
Retatrutide approaches steady state after about 4 to 5 half-lives of regular weekly dosing, which is roughly 24 to 30 days, going by the general rule of pharmacokinetics. Steady state means the amount cleared between administrations equals the amount added, so blood levels stop rising from one week to the next.
This is a rule of thumb, not a measured value. The phase 1b paper did not publish a time to steady state for retatrutide. It reports that once-weekly dosing allowed meaningful steady-state exposure to be reached and maintained. The 4 to 5 half-life rule comes from standard pharmacokinetic teaching.
For laboratory work, lot data and the Certificate of Analysis for each batch are on the retatrutide product page.
How does the retatrutide half-life compare with semaglutide and tirzepatide?
Retatrutide has a half-life of about 6 days, which places it between tirzepatide at about 5 days and semaglutide at about 1 week. All three peptides carry a fatty acid side chain, and all three use a once-weekly interval: retatrutide in trials, semaglutide and tirzepatide under their FDA labels. The table lists each value and its source.
The retatrutide value comes from a 12-week phase 1b trial in type 2 diabetes. The other two come from approved FDA labels. A gap of one day should not be over-read.
Sources
- Urva S et al. The Lancet, 2022;400:1869-81 (phase 1b, LY3437943). doi.org
- Coskun T et al. Cell Metabolism, 2022;34:1234-1247. doi.org
- Eli Lilly, phase 2 protocol J1I-MC-GZBF (NCT04881760). cdn.clinicaltrials.gov
- Jastreboff AM et al. NEJM, 2023 (phase 2, obesity). gwern.net
- MOUNJARO (tirzepatide), FDA prescribing information (DailyMed). dailymed.nlm.nih.gov
- OZEMPIC (semaglutide), FDA prescribing information (DailyMed). dailymed.nlm.nih.gov
- WEGOVY (semaglutide), FDA prescribing information (DailyMed). dailymed.nlm.nih.gov
- Certara: Understanding steady state pharmacokinetics. www.certara.com
Frequently asked questions
Is the retatrutide half-life the same as how long it works?
No. The half-life of about 6 days only describes how fast retatrutide is cleared from blood plasma. How long a biological response lasts in a study is a separate measurement and depends on the model, the endpoint and the amount administered.
Where does the 6-day figure come from?
It comes from the phase 1b trial by Urva and colleagues, published in The Lancet in 2022, which gave once-weekly retatrutide to adults with type 2 diabetes for 12 weeks. An earlier single-dose study in healthy volunteers found a mean of about 5 to 7 days.
Is there an approved dose of retatrutide?
No. Retatrutide is investigational and is not approved by the EMA, the FDA or any other regulator, so no approved dose exists anywhere. Trial schedules only describe what investigators administered in named trial arms. RetaNord supplies retatrutide for research use only.